1. Field of the Invention
The present invention relates to ion implantation, and more especially, to −low temperature ion implantation.
2. Background of the Related Art
Low temperature ion implantation is a new application of ion implantation. It has been discovered that a relatively low wafer temperature during ion implantation is advantageous for the formation of shallow junctions, especially ultra-shallow junctions, which are more and more important for continued miniaturization of semiconductor devices. Besides, it also has been proven to be useful for enhancing the yield of the ion implantation.
At the start of the current low temperature ion implantation, a wafer is moved from an environment, such as an atmosphere environment, into an implanter. As shown in FIG. 1a, before the implant process is started, a cooling process (from tc to ti) cools the wafer temperature from an environment temperature (TR) of about 15˜25° C. to a prescribed implant temperature (TP) (or to be essentially equal to the prescribed implant temperature), which usually is lower than the freezing point of water and generally is about −15˜−25° C. As usual, the prescribed implant temperature represents the setting temperature of an E-chuck, which is used to hold the wafer. Herein, the wafer can be cooled at least in a cassette outside the implanter, in a loadlock chamber of the implanter, in a chamber of the implanter, and so on.
In general, as shown in FIG. 1b, a backside gas with a constant pressure (P0) is applied to cool the wafer and it requires several seconds (even minutes) to cool down the wafer from the environment temperature (TR) to the prescribed implant temperature (TP). After that, the backside gas with the constant pressure is still applied to cool the wafer during the implant process. Referring still to FIG. 1a, during the implant process (from ti to th) of the wafer, the wafer is heated by the ion beam energy and cooled by a cooling mechanism, such as a backside cooling gas. Usually, to keep the wafer to have appropriate implant quality in the low temperature ion implantation, the pressure of the backside gas is properly adjusted to ensure the wafer temperature is always equal to the prescribed implant temperature (TP) or at least is not higher an upper-limited temperature (TL) during the implant process. Herein, the rise curve of the wafer temperature during the implant process (from ti to th) may be linear or non-linear, and the rise curve shown in FIG. 1a is only a sketch. On the other hand, if the upper-limited temperature (TL) is close to the prescribed implant temperature (TP), as shown in FIG. 1a′, the wafer temperature during the implant process (from ti to th) may be thought of as constant.
After finishing the implant process, referring still to FIG. 1a or FIG. 1a′, a heating process (from th to tf) proceeds to heat the implanted wafer until the wafer has an environment temperature (TR′). Herein the environment temperature (TR′) may correspond to the atmosphere environment temperature, so that the difference between the implanted wafer temperature, which will be moved out of the ion implanter immediately, and the environment temperature is decreased. Hence, owing to the decreased temperature difference, the water condensation problem on the wafer surface induced by the temperature difference may be avoided or minimized.
In general, after the implant process, the implanted wafer is transferred into a loadlock chamber in a vacuum state to proceed with the heating process. As usual, to avoid any potential contamination, the implanted wafer is not heated by an active heat source but is heated by thermal radiation between the wafer and the load-lock chamber walls. After the temperature of the implanted wafer reaches the environment temperature (TR'), the load-lock chamber is vented and the implanted wafer is moved outside the loadlock chamber immediately. Herein, owing to the low efficiency of the radiation heat transfer mechanism, it requires some seconds (even some minutes) to heat up the implanted wafer from the prescribed implant temperature (TP) to the environment temperature (TR′).
Therefore, both the cooling process and heating process are time-consuming, so that the wafer throughput of the low temperature ion implantation is limited.